The present invention relates to a lighting device for a hot cathode fluorescent lamp used, for example, for a back light of a liquid crystal display and the like.
In a conventional lighting device for a hot cathode fluorescent lamp used for a back light of a liquid crystal display, starting up the lamp is effected without pre-heating a filament of the lamp in order to light the lamp as disclosed in Japanese Patent Laid-open No. 63(1988)-202841 publication.
As a typical device which uses such a lighting device as described above, there can be mentioned one of which display has a small image plane as in a liquid crystal television.
The reasons why starting up the lamp is effected without pre-heating in such uses as described above are that first, the need of reducing the cost is greatly desired and a pre-heat circuit does not thereby render the reduction in cost possible, and secondly, the need of reducing consumption wattage is greatly desired.
The aforemention second reason will be further described. As a premise, a liquid crystal television is driven using a battery having a small capacity so that the consumption wattage is limited to approximately 3 Watts at the maximum.
For this reason, when a pre-heat circuit is added to the aforementioned conventional lighting device, the pre-heat circuit consumes power similarly to the case during lighting even if the back light is being lighted, thus resulting in a loss of power.
A lamp current is practically of the order of 20 to 20 mA in order to suppress the consumption wattage of the liquid crystal television to approximately 2 to 3 Watts. If a configuration is employed in which a pre-heat circuit is provided to pre-heat a filament even during lighting of a lamp, a value of pre-heat current assumes a value close to a current value of the lamp, which has actually no sense to cause Pre-heating. In other words, the design of causing Pre-heating sufficiently occur becomes difficult if consumption wattage is limited.
Further, in order that disconnection of a lamp filament is made difficult to occur to attain a long life of the lamp, it is necessary to increase a quantity of oxide to be coated on the filament. As a preferable means, there is a means wherein a tungsten coil used as a filament coil is formed into a triple coil to increase a surface area of coating to thereby increase the quantity of oxide to be coated.
However, there is a problem in that if a triple coil is used, a pre-heat current increases as compared with a double coil. Therefore, the triple coil has not been put to practical use as a filament of a back light used for applications such as a liquid crystal television for which consumption wattage is restricted.
As described above, since the conventional lighting device for a hot cathode lamp employs a system for causing lighting without pre-heating a filament of a lamp, there is an advantage that the configuration of a portion of the light circuit out of the lighting device can be obtained simply.
However, there is a problem in that since a filament receives a strong ion impact at the time of starting up the lamp, the filament is apt to break, and accordingly, the life of the lamp is shortened.
Namely, when the lamp is lighted without pre-heating the filament, a discharge passes through a glow-discharge area when the lamp is lighted. Electrons supplied from a cathode for maintaining the discharge of the lamp at that time are compensated for by electrons emitted in a manner that electrodes are hammerred by ions accelerated by a high voltage.
Therefore, a tungsten filament is impacted by ions so that the filament gradually reduces its diameter, finally snapping the filament.